The invention relates generally to mortar compositions and, more particularly, to a refractory mortar composition useful in joining fired refractory shapes in sliding gate valves. Sliding gate valves are used to control the flow of molten steel from a metallurgical vessel, such as a tundish, to a continuous casting mold or molds. A typical slide gate plate, as shown for example in U.S. Pat. No. 4,415,103 to Shapland et al., comprises a plurality of refractory plates, each having a teeming orifice therethrough. When one of the plates is moved out of vertical alignment with the others, the teeming orifices, likewise, move out of register, permitting the flow of molten steel to be selectively throttled between a fully opened to a fully closed condition.
Heretofore, in order to increase the erosion resistance of conventional carbon bonded alumina graphite slide gate plates, a pressed and fired insert of oxide bonded zirconia has been employed in the area surrounding the teeming orifice. The zirconia insert is conventionally cemented into a recessed region formed in a bearing surface of the alumina graphite plate using an alumina, phosphate bonded mortar. Both the zirconia insert and alumina graphite plate are usually tar impregnated prior to the mortar cementing step. The impregnated carbonaceous material sometimes causes bonding problems with conventional alumina mortar. It has also been observed that molten steel contacts the mortar joint during throttling and closing of the valve, causing erosion and corrosion of the alumina mortar and eventual liquid steel penetration of the joint. This problem naturally creates serious safety and maintenance concerns and also affects the overall economics of the continuous casting operation which depends upon long uninterrupted casting campaigns for maximum efficiencies.
The present invention is directed to an improved mortar composition which solves the problems heretofore encountered in joining a zirconia insert to an alumina graphite slide gate plate.
The refractory mortar of the present invention provides improved erosion and corrosion resistance to molten steel while also providing improved bonding adhesion between the carbon of the tar impregnated components than heretofore possible when utilizing conventional refractory cements or mortars.
In addition, the present invention provides a refractory mortar having improved thermal expansion characteristics resulting in better sealing of the joint between the zirconia insert and the alumina graphite slide gate plate during high temperature service.